When a spardeck frigate could trouble a ship of the line

Were two-deck spardeck frigates more useful than ships of the line?

Only in some conditions. Their shallower hull and heavy broadside could create tactical advantages, but they remained smaller and could not replace a line-of-battle ship generally.

A spardeck frigate combined a long-gun deck with another level of carronades, giving it a powerful short-range broadside. Its hull generally sat shallower than a ship of the line and could be more useful in some waters or weather. A well-handled third rate might still out-sail a heavy frigate in rough conditions, but the difference in broadside was not always as great as the rating implied. Positioning could therefore let the frigate inflict serious damage without making it a universal substitute for the larger ship.